1. Field of the Invention
The present invention relates to a method and a device to compensate for a dip in the output voltage of a motor-vehicle alternator.
2. Description of the Related Art
The electrical system of a motor vehicle is supplied with energy by an alternator whose output voltage is regulated by an alternator regulator to a predefined value, e.g., to 14V. The connecting of loads or speed changes in the motor vehicle may lead to a dip in the vehicle voltage. The alternator regulator corrects this voltage dip, which means the torque demand on the drivetrain or on the engine increases. Due to the connecting of the load, a higher torque is needed to drive the alternator. The regulation is carried out by increasing the excitation current in the rotor of the alternator device. This increase is brought about by an increase in the duty factor of a PWM signal set by the alternator regulator. The voltage induced in the stator is increased by the change in the magnetic field resulting from the excitation current.
In the case of a rapid correction of a voltage dip, a correspondingly rapid rise also comes about in the torque-determining excitation current. Particularly at low speeds, the engine is often unable to bring up the torque sufficiently rapidly. In this case, the engine is decelerated by the alternator. This may cause the engine to stall.
To prevent such a stalling of the engine, it is already known to limit the correction rate of an alternator regulator in the lower speed range. This “load response function” limits the rate of change of the duty factor when it must be increased because of a voltage dip. The rate of change in the torque is thereby influenced to the same extent, so that the engine has more time to react to the increased torque demands. However, this procedure has the disadvantage that a corresponding delay thereby occurs in correcting the voltage dip. The voltage dip lasts longer.
If only small loads are connected, and thus the duty factor only has to be increased slightly, then the torque is also increased only slightly, which as a rule, is not critical for the engine. So as not to have to put up with a voltage-dip correction which is slow owing to the load response function in the case of such small load changes as well, it is already known to initially increase the duty factor by a predefined value, also known as “blindzone” or “blind zone”, so that the reaction to the voltage dip is rapid, and only afterwards to further increase with the rate of rise limited by the load response function.
The “blindzone” or “blind zone” is defined as the duty factor delta, within which a rapid correction can take place. In the case of present-day regulators, the “blind zone” is set to a fixed value or may be switched between two values. The possible loads thereby resulting, which may be corrected quickly, as well as the possible changes in torque ensuing from them, are not constant, but rather, are a function of the specific working point of the alternator. Reasons for this dependency on the working point are, for example, the properties of the excitation winding (resistance and inductance) alterable by temperature, as well as the disproportionality of the excitation current to the alternator current, which is caused by the saturation behavior of the iron core of the rotor.
Published German patent application document DE 199 05 984 A1 describes a control device for a motor-vehicle alternator which is driven by an internal combustion engine and, after a rectification of the output voltage generated by it, charges a battery. This control device has a voltage regulator which controls the charging voltage of the battery to a constant value, and a device which gradually raises the current of the alternator after an electrical load has been switched on. The increase amount of a control current of a power circuit-breaker for controlling the excitation current of the alternator at an instant directly after the electrical load is switched on, but prior to the activation of the device for controlling a gradual excitation, is set as a function of the power-generation conditions of the alternator. In particular, the increase amount of the control current of the power circuit-breaker for controlling the excitation current of the alternator is set as a function of the current of the alternator in such a way that the increase amount is raised in response to great alternator current, and lowered in response to small alternator current. As an alternative, the increase amount of the control current of the power circuit-breaker for controlling the excitation current is set in such a way that it is proportional to the excitation-current value prior to the electrical load being switched on. In this way, given the use of the same control device for alternators having different alternator-output capacities, the output current of the alternator in question, which represents a one-time and immediate response to a switching-on of a load prior to the activation of the control of a gradual excitation that increases the alternator current gradually, is held essentially constant.